To the Editor.—In their letter of December 1976, Glickman and Rapp described capillary microscopy findings consistent with the existence of a psoriatic angiogenic factor.1 We read the letter with enthusiasm because we have also recorded experimental observations compatible with that hypothesis. Our enthusiasm was, however, tempered by caution, for proving the existence of any pharmacological "factor" is an annoyingly complex affair.Folkman has isolated a diffusible tumor angiogenic factor (TAF) that is ostensibly essential for the growth of malignant tumors.2 We have used the hamster cheek-pouch technique to confirm the presence of TAF activity in human skin tumors3 and have demonstrated and partially characterized an epidermal angiogenic factor (EAF),4 which is secreted by physiologically and histologically normal hamster and human epidermis. Subsequently, several putative angiogenic substances have been reported in association with lymphocytes, macrophages, and platelets. The precise role(s), even the very existence, of